The embodiments described herein relate generally to solar power generation, and more specifically, to methods and systems for operating a two-stage power converter.
Solar energy has increasingly become an attractive source of energy and has been recognized as a clean, renewable alternative form of energy. Solar energy in the form of sunlight may be converted to electrical energy by solar cells. A more general term for devices that convert light to electrical energy is “photovoltaic cells.” Sunlight is a subset of light. Thus, solar cells are a subset of photovoltaic (PV) cells. A PV cell comprises a pair of electrodes and a light-absorbing PV material disposed therebetween. When the PV material is irradiated with light, electrons that have been confined to an atom in the PV material are released by light energy to move freely. Thus, free electrons and holes are generated. The free electrons and holes are efficiently separated so that electric energy is continuously extracted. Current commercial PV cells use a semiconductor PV material, typically silicon.
In order to obtain a higher current and voltage, solar cells are electrically connected to form a solar module. In addition to a plurality of solar cells, the solar module may also include sensors, for example, an irradiance sensor, a temperature sensor, a voltage meter, a current meter, and/or a power meter. Solar modules may also be connected to form a module string. Typically, the DC voltages output by the module strings are provided to a grid inverter, for example, a DC to AC voltage inverter. The DC to AC voltage inverter converts the DC voltage to three-phase alternating current (AC) voltage or current. The three-phase AC output from the DC to AC inverter is provided to a power transformer, which steps up the voltage to produce a three-phase high-voltage AC that is applied to an electrical grid.
Electricity applied to the electrical grid is required to meet grid connectivity expectations. These expectations address safety issues as well as power quality concerns. For example, the grid connectivity expectations include operating the power generation system during a transient event, for example, a power surge or power failure along the electrical grid. This capability may be referred to as low voltage ride through (LVRT) or zero voltage ride through (ZVRT). An LVRT/ZVRT event is a condition where the alternating current (AC) utility voltage is low on either one phase of the electrical grid or multiple phases of the electrical grid. During an LVRT/ZVRT event, the capacity of the electrical grid to accept power from the power generation system is low. Another grid connectivity expectation is that the generated power be conditioned to ensure that the power matches the voltage and frequency of the electricity flowing through the electrical grid.